1. Field of the Invention
The present invention relates to an exposure apparatus and a method of manufacturing a device.
2. Description of the Related Art
Along with the recent micropatterning of semiconductor integrated circuits, such as an IC and an LSI, a demand has arisen for further improving the resolution, overlay accuracy, and throughput of a projection exposure apparatus. A problem in terms of the imaging characteristics of exposure light has conventionally been posed. That is, a lens of a projection optical system thermally deforms and changes in refractive index upon absorbing exposure light, to result in aberration, as an error of the optical characteristic of the projection optical system.
The projection magnification of the lens of the projection optical system sometimes changes for each shot region of a substrate (also called a wafer) to result in aberration. To solve this problem, Japanese Patent Laid-Open No. 10-199782 discloses a technique of calculating an amount of lens driving corresponding to an amount of projection magnification change, for each shot region, based on projection magnification data, and driving the lens of the projection optical system in the optical axis direction. This makes it possible to adjust or to correct the projection magnification.
Even during non-exposure, the lens of the projection optical system sometimes deforms due to the influence of heat absorbed upon exposure, to result in aberration during the next exposure. To prevent this problem, driving the lens of the projection optical system at the start of the next exposure makes it possible to adjust or to correct aberration due to its thermal deformation.
From the viewpoint of the imaging characteristics of the exposure light, it is becoming difficult to tolerate aberration that occurs due to even a slight change in atmospheric pressure. Under an environment in which the exposure apparatus is installed, the atmospheric pressure sometimes changes rapidly, albeit, only slightly, due to a human factor, to result in aberration. To solve this problem, Japanese Patent Laid-Open No. 2001-085292 discloses a technique of coping with a case wherein the atmospheric pressure changes about 0.5 hPa/sec upon opening/closing a door of a clean room. In this case, a barometer using a laser interferometer with good response detects a change in atmospheric pressure to drive the lens of the projection optical system or to change the wavelength of light from an exposure light source, in accordance with the change in atmospheric pressure. This makes it possible to adjust or to correct aberration due to a change in atmospheric pressure.
In general, aberrations that occur upon a change in atmospheric pressure are very large, relative to projection magnification and focus, and, hence, need to be adjusted or reduced. On the other hand, aberrations of, for example, distortion, sphere, coma, astigmatism, and curvature of field are relatively small, and, hence, cannot pose any problems in terms of the exposure characteristic.
Assume that an alignment measurement system of the TTL (Through The Lens) scheme is used for measurement (to be referred to as alignment measurement hereafter), for alignment between a reticle (also called a mask or original) and a wafer. Driving the lens of the projection optical system immediately before the start of alignment measurement makes it possible to reduce aberration that occurs during alignment measurement.
In the techniques described in Japanese Patent Laid-Open Nos. 10-199782 and 2001-085292, aberration is corrected in the interval of shot exposure, during shot exposure, at the start of exposure, or at the start of alignment measurement.
However, Japanese Patent Laid-Open Nos. 10-199782 and 2001-085292 do not disclose any method of correcting aberration during a period (non-exposure period) from the end of exposure until the start of the next exposure, such as a wafer exchange period, reticle exchange period, or periodical maintenance period. If, for example, the non-exposure period is long, the amount of driving of the lens of the projection optical system, for aberration correction, is sometimes calculated based on data of an exposure period immediately before the start of exposure and data of the non-exposure period. Since the lens of the projection optical system tends to be driven based on the thus calculated lens driving amount information, a relatively large amount of driving becomes necessary for the lens of the projection optical system during the exposure period. This often requires a long settling time after driving the lens of the projection optical system. Consequently, the throughput of processing by the exposure apparatus may be lowered.
The atmospheric pressure sometimes changes greatly during the non-exposure period, such as the wafer exchange period, reticle exchange period, or periodical maintenance period. Even in this case, the amount of driving of the lens of the projection optical system, for aberration correction, is sometimes calculated based on atmospheric pressure data immediately before the start of exposure. Since the lens of the projection optical system tends to be driven based on the thus calculated lens driving amount information, a relatively large amount of driving amount becomes necessary for the lens of the projection optical system during the exposure period. This often requires a long settling time after driving the lens of the projection optical system. Consequently, the throughput of processing by the exposure apparatus may be lowered.
If aberration is not corrected, to suppress a decrease in the throughput of processing by the exposure apparatus, the exposure accuracy may become lower.
In some cases, aberration is corrected only at the start of alignment measurement during a period (alignment measurement period) for alignment measurement. Even after the start of alignment measurement, however, aberration may occur when a long period of alignment time is spent, to improve the overlay accuracy or when the atmospheric pressure changes greatly during alignment measurement. The aberration sometimes causes an alignment measurement error. This may similarly result in a decrease in exposure accuracy.